Deformation experiments were performed on glaucophane and tremolite powders, respectively, using a DDIA apparatus mounted on a synchrotron beamline at the APS. Preliminary results demonstrate that glaucophane, when deformed metastable under P-T conditions approx. 3 GPa, 500 °C, exhibit stick-slip behaviour (http://en.wikipedia.org/wiki/Stick-slip_phenomenon). Each stress drop can be correlated with an acoustic emissions that was captured during the experiment, and inferred to come from the sample assembly. The expected mineral reaction during the experiment is glaucophane ↔ 2 jadeite + talc (Figure 1). We used natural glaucophane crystals which never show pure endmember composition so the real reaction happened during deformation will be more complex. Nevertheless we deformed the sample in the jadeite and talc stability field (Figure 2) and the video of the X-ray diffractograms (http://en.wikipedia.org/wiki/Powder_diffraction), which were taken in situ of the sample, proof that talc and also jadeite are growing during the experiment (diffraction video and Figure 3.
Figure 1: P-T pseudosection based on pure glaucophane endmember composition. This pseudosection was calculated using Perplex
Figure 2: P-T pseudosection based on natural glaucophane from Corsica. The pseudosection was calculated using Theriak Domino.
Figure 3:The diffractogram on the left was taken before the deformation and the one on the right was taken afterwards. It can clearly be seen, that talc (blue) and jadeite (turquoise) were growing during the deformation of the glaucophane sample.
The observation of high pressure stick-slip was also confirmed on a pure tremolite (a calcic amphibole, from Balmatt, NY) mineral assembly, which also exhibited stick-slip behaviour under 2-3 GPa. The underlying mineral reactions which took place during this experiment is tremolite ↔ 2 diopside + talc (Figure 4).
Figure 4: P-T pseudosection based on pure tremolite endmember comopsition. Calculated using Perplex.
Microtomograph analyses of the glaucophane and the tremolite samples show that there are many faults crosscutting the samples (videos).
During a third experiment, performed at higher temperature where the expected reactions are lawsonite and glaucophane dehydrations under eclogite facies conditions (Figure 5), no stick-slip behaviour was observed, but acoustic emissions were nevertheless detected at low temperature.
Figure 5: P-T pseudosection based on the bulk rock chemistry of a blueschist from northern Corsica. In blue the dehydration reaction of glaucophane and in orange the dehydration of lawsonite. The pseudosection was calculated using Theriak Domino.
The fact that the high pressure amphibole breakdown reactions of tremolite and glaucophane generated mechanical instabilities which lead to little earthquakes in the laboratory and the dehydration reactions did not, suggest, that these mineral reactions could potentially play an important role in the generation of intermediate earthquakes.
Additional experiments, both in Griggs and DDIA apparatus, will be performed in the near future in order to confirm our preliminary results. Microstructural analysis using SEM, Raman and EMPA is also planned.